1. Field of the Invention
The present invention relates to an outboard motor having a mechanism for transmitting engine power to a propeller through a shaft, and especially relates to an outboard motor equipped with a transmission for changing the rotational speed of a shaft.
2. Description of the Related Art
Conventionally, in an outboard motor, engine power is transmitted to a crankshaft, a drive shaft, and a propeller shaft, and then is transmitted to a propeller from the propeller shaft. Conventionally, the engine power is transmitted as is to the propeller through each of the aforementioned shafts. Thus, when it is desired to change the speed of a watercraft, the engine power is adjusted to change the rotational speed of each shaft, so that the rotational speed of the propeller is changed subsequently to cause a change in the speed of the watercraft.
However, recent outboard motors have been provided with a transmission on a drive shaft, and are adapted to change the speed of a watercraft with the transmission rather than relying exclusively on the engine power (see WO 2007/007707, for example).
It is often the case that a conventional outboard motor has a plurality of mounts for supporting and mounting the outboard motor to a hull, and usually has two upper mounts and two lower mounts. Of these mounts, the two upper mounts are often disposed on a mounting plate provided above an upper casing that supports a drive shaft therein while having a short distance therebetween.
However, when the two upper mounts of the outboard motor are disposed on the mounting plate provided above the upper casing while having the short distance therebetween, the mounting plate as well as an upper section of the upper casing that is joined to the mounting plate become narrow in width. However, a transmission that is mounted on the drive shaft and disposed in the upper casing has a considerable width. Thus, if the upper section of the upper casing is narrowed, the transmission cannot be inserted from above the upper casing during installation of the transmission in the upper casing. Consequently, problems arise such as a complicated structure of the upper casing and a troublesome assembly work. The transmission has to be inserted from another insertion opening such as one separately provided for the transmission in order to dispose the transmission in the upper casing. In addition, it may happen that the transmission has to be mounted on the drive shaft after being inserted in the upper casing.
In addition, in the outboard motor in which the upper mounts and the lower mounts are disposed on different members such that the upper mounts are disposed on the mounting plate and the lower mounts are disposed on the upper casing, first, the outboard motor has to be detached from the hull along with the mounting plate and the upper casing in order to remove the transmission from the outboard motor during maintenance of the transmission. Then, after the outboard motor is detached from the hull, the transmission is removed for the maintenance. This causes a problem of complicating the maintenance work.
Furthermore, conventionally, in an outboard motor, the engine, the mounting plate, and the upper casing are jointly secured by a through bolt from the upper casing side. Still further, the mounting plate and the upper casing are secured by another bolt. As described above, among the conventional outboard motors, there are motors that are supported and mounted to the hull by the upper mounts disposed on the mounting plate and by the lower mounts disposed on a lower section of the upper casing.
In the outboard motors having such a construction, when only the engine is disassembled for maintenance, the engine can be removed by unscrewing the through bolts while the outboard motor remains supported and mounted to the hull by each of the mounts. However, when the through bolts are used, the positions of the through bolts are determined according to the exterior shape of the engine. Therefore, it is difficult to enlarge only the upper casing, for example.